Journal article

Hyperbranched Anatase TiO2 Nanocrystals: Nonaqueous Synthesis, Growth Mechanism, and Exploitation in Dye-Sensitized Solar Cells

A colloidal crystal-splitting growth regime was used in which TiO2 nanocrystals, selectively trapped in the metastable anatase phase, can evolve to anisotropic shapes with tunable hyper-branched topologies over a broad size interval. The synthetic strategy relies on a nonaq. sol-gel route involving programmed activation of aminolysis and pyrolysis of Ti carboxylate complexes in hot surfactant media via a simple multi-injection reactant delivery technique. Detailed studies indicate that the branched objects initially formed upon the aminolysis reaction, possess a strained monocryst. skeleton, and their corresponding larger derivs. grown in the subsequent pyrolysis stage accommodate addnl. arms crystallog. decoupled from the lattice underneath. The complex evolution of the nano-architectures is rationalized within the frame of complementary mechanistic arguments. Thermodn. pathways, detd. by the shape-directing effect of the anatase structure and free-energy changes accompanying branching and anisotropic development, are considered to interplay with kinetic processes, related to diffusion-limited, spatially inhomogeneous monomer fluxes, lattice symmetry breaking at transient Ti5O5 domains, and surfactant-induced stabilization. Finally, as a proof of functionality, the fabrication of dye-sensitized solar cells based on thin-film photoelectrodes that incorporate networked branched nanocrystals with intact crystal structure and geometric features is demonstrated. An energy conversion efficiency of 6.2% was achieved with std. device configuration, which exceeds the best performance with prototypes of split TiO2 nanostructures. Anal. of the relevant photovoltaic parameters reveals that the used branched building blocks indeed offer light-harvesting and charge-collecting properties that can overwhelm detrimental electron losses due to recombination and trapping events.


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